1. Field of the Invention:
The present invention relates to an underwater positioning system for establishing the position of submerged moving bodies such as deep-sea tow devices and the like, or objects such as underwater piston corers and sunken ships. It particularly relates to a global positioning system or differential global positioning system (both of which are also collectively referred to herein as global positioning system, or GPS), as well as underwater object positioning systems that are a combination of acoustic positioning systems and Loran C and microwave-based short-range positioning systems.
2. Description of the Prior Art:
As shown in FIG. 11, a conventional long baseline (LBL) system for determining the position of a submerged moving object uses three acoustic transponders 4 that are placed at prescribed locations on the seabed. A transducer 3 is installed on the bottom of a workboat 2. The direct distance from the transducer 3 to each of the acoustic transponders 4 is measured by measuring the time it takes for an acoustic wave to travel from transducer 3 to acoustic transponder 4 and back to transducer 3, which is used to establish the position of the transducer 3 within the coordinate system formed by the acoustic transponders 4. By then measuring the direct distance from the submerged moving object 1 to the acoustic transponders 4, and the distance from the moving object 1 to the transducer 3 of the workboat 2, the position of the moving object 1 is established in terms of the coordinate system formed by the acoustic transponders 4.
In the conventional super-short baseline (SSBL) system, a transducer affixed to the bottom or side of a workboat transmits acoustic signals to an acoustic transponder on the submerged moving object, and the time it takes for the transmitted signals to be received is used to determine the direct distance to the object and the direction in which the object is moving.
FIG. 12 shows an improved version of the conventional LBL system. This system comprises an acoustic receiver 6 on a master workboat 2 and acoustic receivers 7 on two auxiliary workboats 5. Here, the submerged moving object 1 is a submersible research vehicle on which there is a synchronous pinger 8. The synchronous pinger 8 transmits pulses that include depth data,. and the pulses are received by the acoustic receivers 7 and used to determine the position of the submersible research vehicle 1.
LBL positioning systems have good positioning accuracy but require the use of at least three acoustic transponders, and have a range of about three or four square kilometers. In order to use such systems for large-area positioning applications, numerous transponders have to be located on the seabed. Moreover, the system has to be calibrated beforehand by determining the depth of the transponders and their relative locations. Thus, the positioning operation is not efficient.
While SSBL systems do not require the provision of multiple transponders or calibration and hence the positioning operation is efficient, they do have the drawback that the shortness of the baseline results in a decrease in accuracy when the system is used to determine positions over long distances.
The improved LBL system shown in FIG. 12 is a large-scale system, involving as it does the use of two auxiliary workboats and people to handle each boat. Moreover, noise generated by the screws of the master and auxiliary vessels can make it impossible to record the necessary measurement readings. Also, the system requires radio equipment to transmit data obtained by the auxiliary workboats to the master vessel, and in foreign waters, regulations on the use of radio transmission may make it impossible to use the system.
An object of the present invention is to solve the drawback of the foregoing conventional underwater object positioning systems and therefore to provide an underwater object positioning system that has good position measurement accuracy but does not require the installation of two or more seabed transponders, does not need to be calibrated, and also does not require auxiliary vessels, so it does not have to be made large-scale in terms of the system equipment involved.
To attain the above object, the present invention provides an underwater object positioning system comprising a workboat, an acoustic transmitter-receiver mounted on the workboat, first and second surface tow units towed by the workboat, an acoustic transducer that is mounted on the first surface tow unit, that is connected by a communication cable to the acoustic transmitter-receiver on the workboat and that is positioned by a radio positioning means, two acoustic receivers that are mounted on the second surface tow unit, that are connected by a communication cable to the acoustic transmitter-receiver on the workboat and that are positioned by a radio positioning means, an acoustic pulse transmitter mounted on a submerged positioning target, and a computing processor mounted on the workboat that computes a location of the submerged target based on positional data on positions of the acoustic transducer and each acoustic receiver, and distance data on distance from the acoustic transducer and acoustic receivers to the acoustic pulse transmitter.
The above radio positioning means can be a global positioning system. The acoustic pulse transmitter can be an acoustic transponder, a synchronous pinger or a synchronous responder. The submerged positioning target includes such objects as deep-sea tow devices towed by a workboat, sunken vessels lying on the seabed, piston corers used to obtain seabed samples, and divers.
In accordance with the above-described positioning system of this invention, first, the radio positioning means is used to determine the locations of the acoustic transducer on the first surface tow unit and the acoustic receivers on the second surface tow unit. The direct distance from the acoustic transducer on the first surface tow unit and the acoustic receivers on the second surface tow unit to the acoustic pulse transmitter on the positioning target is then obtained from the time it takes for a sound wave to traverse the distance. Since the acoustic pulse transmitter will be located at the point at which the three distance lines intersect on a spherical surface taken as a radius, the position of the submerged object can be determined by obtaining the point of intersection.
To simplify the computation and increase the positioning accuracy, the depth of the acoustic pulse transmitter is measured and the three direct distances and the depth are used to obtain each of the horizontal distances from the acoustic transducer on the first surface tow unit and the acoustic receivers on the second surface tow unit to the acoustic pulse transmitter.
In the positioning system according to the present invention, only one acoustic transponder has to be provided on the object concerned, eliminating the need in the case of a conventional LBL system to provide at least three transponders on the seabed and to carry out system calibration. Thus, the positioning range is increased. Moreover, surface tow units are used instead of auxiliary workboats, so the scale of the system equipment can be reduced and the need for people to handle the boats is also eliminated. In addition, since the data acquired by the acoustic transducers and receivers is transmitted to the acoustic pulse transmitter on the master workboat via cable, the system is not subject to radio-related regulations.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.